In particle beam (PB) systems, specimens or “workpieces” are retained on movable stages for positioning relative to the beam. Particle beam systems are used in a plurality of industrial fields, including, but not limited to, testing systems, imaging systems like scanning electron microscopes (SEMs), inspection systems for semiconductor devices, and exposure systems for pattern writing via lithography.
There is a high demand for structuring, testing and inspecting specimens within the micrometer and nanometer scale. Micrometer and nanometer scale process control, inspection or structuring is often done with charged particle beams, e.g. electron beams. Charged particle beams offer superior spatial resolution compared to, e.g. photon beams, due to their short wavelengths. However, there are also light-optical, ultra-violet, and x-ray systems with a stage that supports a specimen and the stage being movable to position the specimen with a precision of 50 nm or better.
Generally, as the precision of measurement, testing, or patterning systems increases there is a demand for high precision and fast positioning of specimen in those systems. The stage holding the specimen is typically independently movable in the x-direction and the y-direction. In conventional systems, positioning data of the stage is measured in two perpendicular axes (e.g., X and Y axes).
Interferometers are often used to detect the position of the stage, as shown in FIG. 5, which illustrates a prior art charged particle beam system. As illustrated, conventional interferometer positioning systems often include three laser beams per axis (including measurement beams and a reference beam that interfere to some degree depending on the relative lengths of the paths traveled). Thereby, the three laser beams a, b, c are guided towards the measurement positions along the axis to be measured through a single monolithic optical element 502.
According to this arrangement, two laser beams b, c are directed to a mirror 22 at a stage 20, whereas one laser beam is directed to a mirror 12 at a charged particle column 10 and is used as the reference laser beam for the interferometric measurement. Thus, the two measurement paths directed to and reflected from the stage interfere with the reference beam directed to and reflected from the column. Thereby, two distances of the stage with respect to the column are obtained.
Such position measurement have historically been widely used, at least in part, because the external reference beam path can significantly reduce the amount of data to be evaluated. As shown in FIG. 6, interferometric optical element 60 directs the reference beam a onto column 10. Measurement beam b is directed onto stage 20. Optical element 60 combines the reflected beams to interfere with each other. The monolithic optical element of 502 (shown in FIG. 5) combines the three beams such that the optical-digital converter 510 only needs to convert the superimposed reflected beams. The conversion results may then be provided to beam controller 522 and/or stage controller 524.
While use of the monolithic multi-axis interferometer 502 may reduce the amount of data to be converted, there may also be a number of disadvantages. For example, monolithic multi-axis interferometers typically have to be precisely pre-aligned, since the monolithic optical system does not allow for individual alignment of the laser beams. This pre-alignment is time consuming and expensive. Also, internal adjustments (e.g., of any particular measurement path) are typically not possible, thereby preventing compensation for internal imperfections in the monolithic optical components. Further, multi-axis interferometers utilizing monolithic optics tend to be heavy and prone to vibration which may reduce the accuracy of position measurements.
Accordingly, what is needed is improved systems for detecting positions of moving stages.